During fetal development, a structure called the foramen ovate of the heart remains open to allow blood from the venous system to bypass the lungs and go to the systemic circulation. This is because prior to birth, the oxygenation of the blood in the fetus is via the placenta and not the lungs. A layer of tissue begins to cover the foramen ovale during fetal development, and typically seals over the foramen ovate soon after birth.
In a certain percentage of adults, however, the foramen ovale does not seal over. As a result, blood can flow directly between the atria of the heart. The blood flow can occur through direct openings between the atria and/or through a flap-like opening in the septum between the atria. In this latter case, elevation of pressure in the pulmonary circulation can cause blood to flow through the flap-like opening of the foramen ovate. This condition is known as a patent foramen ovale (PFO).
Typically, left atrial (LA) pressure is higher than right atrial (RA) pressure. As a result, the flap-like opening of a PFO usually remains closed. Under certain conditions, (e.g., pulmonary hypertension due to various causes, or transiently during a cough) right atrial pressure can exceed left atrial pressure. This pressure difference can create the possibility that blood and/or a blood clot could pass from the right atrium to the left atrium through the opening, allowing the blood clots to enter the arterial circulation. This is known as a paradoxical embolus because the blood clot paradoxially enters the arterial circulation instead of going to the lungs. Once in the arterial circulation, a blood clot could pass to the brain to result in a stroke. It is desirable that the possibility for this event to occur be eliminated.
The illustrations provided in the Figures are not to scale.